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Posted By Topic: 2 way/intermediate circuit inductance

Aug 28 2019 08:14

Yesterday was the second job i've come across where the lighting fixture (low voltage LED) glows or flashes when in the off position due to the 80-90V lurking in the strap wires. Are the lamps or driver just not fit for purpose or is there a common fix for this that i haven't heard of? I'll have a look at the driver specs today.

Aug 28 2019 09:00

It's from capacitive coupling between the conductors. The very low load of the lamp, can use the coupled voltage to glow or flash.

Usually it's more common with poor quality lamps/drovers, but it can happen with any LEDs.

Some people have had success eliminating the problem by putting 1 or more load capacitors across the lamp connection. You can use up to 3 if necessary.

Aug 28 2019 09:40

Happens with CFLs also.
The longer the switch strap, the greater the capacitance between cores, and the more likely it is to happen.

40 years ago it used to happen with(neon)fault indicator lights (eg high level alarm for septic tanks).

Fitting a capacitor to deal with an effect of capacitance may seem counter-intuitive; but since the coupled voltage is a.c. a capacitor forms a path for it to bleed away, instead of building to the point where the lamp driver can use it. A (high value) resistor would do the same job. the result is a tiny leakage current that runs 24/7, but amounts to SFA in a house-sized installation - maybe $2 a year on the power bill.